Anelastic Strain Recovery Measurements Research Articles

Three-dimensional stress state above and below the plate boundary fault after the 2011 Mw 9.0 Tohoku earthquake

The Integrated Ocean Drilling Program conducted Expedition 343 and 343T, named the Japan Trench Fast Drilling Project (JFAST), to drill through the plate boundary fault that ruptured during the 2011 Mw 9.0 Tohoku earthquake in the area with the largest fault slip displacement near the Japan trench. Analyses of breakouts observed from borehole C0019B produced postearthquake stress states above the plate boundary fault between the subducting Pacific plate and overriding North American plate. To supplement the lack of stress data below the rupture zone of the earthquake, we conducted core-based three-dimensional stress measurements by the anelastic strain recovery (ASR) method using four whole-round core samples of sediments, of which three samples were located above, but one sample was located below the plate boundary fault in borehole C0019E. As a result of the stress measurements, the postearthquake three-dimensional stress magnitudes at ∼802 and ∼828 meters below seafloor (mbsf) across the plate boundary fault at ∼820 mbsf reveal a normal faulting stress regime. The differences between the three-dimensional intermediate principal stress and the minimum principal stress at the two depths are less than 1 MPa, suggesting a complete release of horizontal tectonic stresses that accumulated before the earthquake. In addition, the maximum horizontal stress SHmax azimuth N115°E at ∼828 mbsf below the plate boundary fault from ASR measurements shows consistency with the SHmax azimuth N139 ± 23°E (mean ± standard deviation) at ∼550–810 mbsf from breakout analyses above the fault. Taken together with the similar stress magnitudes at ∼802 and ∼828 mbsf, we interpret that the postearthquake stress states are almost the same in the sediments above and below the plate boundary fault. In other words, the stress state in terms of both orientation and magnitude is continuous across the fault. At a shallower depth of ∼177 mbsf in the slope sediments, the ASR stress data reveal a “stress state at rest”, which is likely free from tectonic effects of plate subduction, suggesting that the stress state was reset by the great coseismic displacement of ∼50 m slipped during the Tohoku earthquake.

In-situ stress analysis using the anelastic strain recovery (ASR) method at the first offshore gas production test site in the eastern Nankai Trough, Japan

To evaluate in-situ stresses by applying the anelastic strain recovery (ASR) method to drilling cores, four cores retrieved from different depths between 265 and 313 m below seafloor in a well at the first offshore gas production test site in the eastern Nankai Trough, Japan were utilized to measure anelastic strains onboard the drilling ship. The onsite ASR measurements suggest this depth range in the study area is a normal faulting stress regime dominated by gravitation. In subsequent laboratory experiments, the cores were returned to their in-situ stress states via the K0 consolidation procedure, then elastic and anelastic strains were measured during and after the unloading process to verify the onboard measurements and quantify in-situ stresses. These laboratory calibration experiments verified that the ASR measurements following the stress release could provide reasonable estimates for the artificially applied preload stress.

ボーリングコア側面の非弾性ひずみと初期応力の関係について

At three boring sites, principal strain orientations obtained from anelastic strain recovery on core surface were compared with in-situ stress states determined by overcoring or hydraulic fracturing method. Orientations of time-dependent maximum expansion of a core taken from the vicinity of a large mined goaf indicated that they presumably reflected the maximum stress state when the core was retrieved and they were totally different from the stress orientations determined by overcoring technique. Three dimensional principal strain orientations obtained from anelastic strain recovery on surface of granodiorite cores (150mm in diameter) were in good agreement with the in-situ stress state predicted by overcoring method. And anelastic strain recovery measurements on Mannari granite were carried out immediately after the 38 mm diameter cores were removed from 300 m deep borehole, where none of three principal axes of strain coincided with the borehole axis. Laboratory measurements of compressional velocity on relaxed Mannari cores showed that the direction of the slowest velocity correlated with the one of maximum expansion and the strike of microcracks in quartz crystals. Then it is suggested that three dimensional stress state can be determined by combination of hydraulic fracturing test at single borehole section and observation of anelastic strain variations of the core taken from the borehole.

Significance of Coring-Induced Fractures in Mesaverde Core, Northwestern Colorado (1)

Several types of mechanically induced fractures occur in Mesaverde core from the U.S. Department of Energy's Multiwell Experiment wells. Two of these types of fractures have significance for the determination of in-situ stress orientations. Scribe-line fractures originate at orientation grooves on the surface of the core and are initiated by scribing knives in the core barrel minutes after the core is cut free from the surrounding rock. In sandstone core, these fractures strike parallel to the maximum in-situ horizontal stress direction that has been determined in these wells by anelastic strain recovery measurements. Coring-induced petal and petal-centerline fractures form just below the advancing core bit. Their strike is also controlled by the in-situ stress field. Loc lly, however, strike may be secondarily influenced by shear stress created by bit rotation, reorienting petal fracture strikes in the direction of rotation. Nevertheless, in many situations the orientations of these two types of fractures may be used to determine the orientation of the horizontal in-situ stress field if the core is oriented. Even in unoriented core, the relative orientation between in-situ stress and natural fractures may be determined.

State of stress and relationship of mechanical properties to hydrothermal alteration at Valles Caldera core hole 1, New Mexico

We measured the densities, total and microcrack porosities, and ultrasonic velocities of a number of core samples from an 856‐m‐deep core hole near the Banco Bonito vent at Valles Caldera, New Mexico. Reductions in porosity with depth define a zone from about 600 m down where hydrothermal mineralization and recrystallization have been most active. This zone is also reflected in a large decrease in the anisotropy of acoustic velocities. Stress orientation estimates based on microcrack orientations at the 812‐m depth as determined by differential strain curve analysis, as well as anelastic strain recovery measurements on a sample from 472‐m depth, show a horizontal E‐W minimum compression direction and a maximum compression inclined about 30° from vertical.

Determination of in situ stress from anelastic strain recovery measurements of oriented core: comparison to hydraulic fracture stress measurements in the Rollins Sandstone, Piceance Basin, Colorado : In: Rock Mechanics in Productivity and Protection (paper to the 25th Symposium on Rock Mechanics, Evanston, Illinois, 25–27 June 1984), P176–185. Publ New York: AIME, 1984

A method to determine in situ stress directions and magnitudes from anelastic strain recovery measurements of oriented core has been used to determine the principal horizontal in situ stresses in the Rollins Sandstone in a deep well near Rifle, Colorado. The principal horizontal stress directions were determined directly from the principal horizontal strain recovery directions. The principal horizontal stress magnitudes were calculated from the principal strain recovery magnitudes, overburden stress, and Poisson's ratio of the rock using a viscoelastic model. The accuracy of the in situ stress directions and magnitudes determined from anelastic strain recovery measurements was substantiated by a direct comparison with open-hole hydraulic fracture stress measurements. The anelastic strain recovery method predicted the magnitudes of the maximum and minimum horizontal principal stresses to be 54.5 MPa and 51.9 MPa, respectively; with an azimuth of N63/sup 0/W +- 8/sup 0/ for the maximum horizontal stress. The hydraulic fracture stress measurements yielded maximum and minimum horizontal principal stress magnitudes of 49.6 MPa and 46.8 MPa, respectively. The azimuth of the maximum horizontal stress ranged from N50/sup 0/W to N70/sup 0/W. 17 references, 2 figures, 4 tables.

Prediction of hydraulic fracture azimuth from anelastic strain recovery measurements of oriented core : Teufel, L W Proc 23rd Symposium on Rock Mechanics, Berkeley, 25–27 August 1982, P238–245. Publ New York: AIME, 1982

Prediction of hydraulic fracture azimuth from anelastic strain recovery measurements of oriented core : Teufel, L W Proc 23rd Symposium on Rock Mechanics, Berkeley, 25–27 August 1982, P238–245. Publ New York: AIME, 1982